SMS FeatureNotAvailable in Sandbox

Getting a FeatureNotAvailable error in our sandbox app. Happens in Postman and .NET. The sandbox number is "+16506849983".

Request:

{
    "to": [
        {
            "phoneNumber": "*****"
        }
    ],
    "from": {
        "phoneNumber": "+16506849983"
    },
    "text": "Test SMS message"
}

Response:

{
    "errorCode": "FeatureNotAvailable",
    "message": "The requested feature is not available",
    "errors": [
        {
            "errorCode": "MSG-242",
            "message": "The requested feature is not available"
        }
    ]
}